1. Field of the Invention
The invention relates to a method and a device for color decoding, and to a television signal display apparatus (TV set, PC having TV signal processing functions, etc.) comprising such a color decoding device.
2. Description of the Related Art
Until now, multi-standard (PAL/NTSC) color decoding, without digitizing the chrominance signal, has mainly been done using an analog phase-locked loop, in which a voltage-controlled crystal oscillator (VCXO) is locked to the color burst signal, see J. van Lammeren et al., Multi-Standard Video Front End, IEEE Transactions on Consumer Electronics, Vol. 37, No. 3, August 1991, pp. 190-196. A simplified diagram of a prior art analog color decoder is given in FIG. 1. The voltage controlled crystal oscillator VCXO regenerates sine and cosine versions of the color subcarrier, both with the correct phase. The subcarrier signals are fed to analog multipliers, where they are multiplied with the chrominance signal. By these multiplications, the chroma quadrature components U and V are separated and demodulated. The main disadvantage of this type of color decoder is that for each variant of the PAL/NTSC standards, a different external crystal is necessary. Therefore a conventional multi-standard color decoder IC has to be equipped with several external crystals which can be tuned satisfactorily, and thus with several additional IC pins.
In Murayama et al., Single-Chip BICMOS Multistandard Video Processor, IEEE Transactions on Consumer Electronics, Vol. 42, No. 3, August 1996, pp. 739-749, a color decoder is described that uses only one external crystal for decoding all variants of the PAL/NTSC standards. However, that system has some disadvantages:
The external crystal that is used in this system must still be a crystal which can be tuned satisfactorily. Tunable crystals are much more expensive than standard crystals, which are poorly tunable.
As the crystal oscillator (VCXO) in this system is locked to the incoming color burst, it can not be used, at the same time, as an asynchronous clock generator for other on-chip applications, e.g., teletext decoding or A/D and D/A conversion.
The system contains an analog PLL that acts as a bandpass filter for the digitally generated subcarrier. The performance of the VCO in this PLL completely determines the overall quality of the color decoder. The required specifications for this VCO can only be achieved when using a well-characterized process (e.g., BiCMOS) and will be degraded by the presence of digital circuitry on the chip, which causes substrate noise. As there is a trend towards CMOS processes and more and more digital functionality, this color decoder is not fully `future proof`.
The automatic phase control (APC) loop still requires an external loop filter, which necessitates an extra IC pin.